"In the 15th century, Samurai began to be equipped with plate armor which provided some limited protection from the early muskets which appeared at the same time. (Eventually firearms' increasing penetrative ability would render all personal armor useless, of course.) The added armor made the Samurai even more impervious to improvised peasant weaponry, increasing their control over the masses around them."

If used in large groups; Heavy Samurai can be devastating and can destroy a base easily; if not in heavy fire.

Catapults are devastating against Heavy Samurai and can kill a group of them in a few hits if in the range of the impact. Spread your Heavy Samurai around so the catapult would have to hit each Heavy Samurai to kill it and that its impact would not affect other Heavy Samurai as well or deploy a few heavy cavalry to destroy it.

Use ranged infantry such as crossbowmen to support Heavy Samurai.

Heavy Samurai can be used as a distraction for many defenses and can help protect ranged infantry such as crossbowmen from heavy fire from defenses.

Samurai were an elite warrior class in ancient japan, roughly with the same status and purpose as knights in feudal Europe.

Samurai were ruled by a code called Bushido.

Although they are traditionally thought of as swordsmen, Samurai actually started as elite bowmen. Eventually, as they became a permanent warrior class they also began training and fighting with all kinds of weapons, including yari (spears), different kind of swords like the katana and the no dachi (a very long sword), tanegashima (muskets) and even blunderbusses.